Exhaust-driven turbochargers are themselves well known in the automotive arts. The idle speed of an exhaust-driven turbocharger often exceeds 30,000 rpm with top speeds of 90,000 to 120,000 rpm. When the automobile engine is operating, lubrication is supplied to the turbocharger's center bearing by the engine's lubrication system. When the engine shuts down, the flow of oil to the turbocharger center bearing essentially ceases. A time period of about 20 to 30 seconds can elapse between the time that the engine shuts down and the time at which the turbocharger stops turning. During this time period, the center bearing temperature can increase to as high as 1200.degree. F. At such high temperatures, and because of the lack of oil flow, the small amount of oil remaining in the turbocharger center bearing carbonizes and ultimately the bearing will fail as a result of galling or seizure.
The use of prelubricating devices and oil reservoirs or accumulators of various designs in conjunction with internal combustion engines are generally well known. These systems are used to prevent engine wear during starting of an internal combustion engine after it has been shut down for a sufficient time to allow its lubricating oil to drain into the engine's oil pan and crank case. This leaves many vital engine parts with no lubricant protection until the engine has been started and the oil pressure brought up to an acceptable level by the oil pump. Various preoiling devices have been designed, all having the purpose of providing oil pressure to the engine prior to start up.
My prior U.S. Pat. No. 4,094,293 issued June 13, 1978 discloses an engine preoiler and lubricant reservoir assembly comprising a hollow cylinder divided into two chambers by a slidable piston. U.S. Pat. No. 4,094,293 also discloses a highly advantageous solenoid controlled valve assembly which regulates the flow of lubricating fluid out of the oil reservoir when the engine is started up or in instances when oil pressure is low and conversely, into the reservoir during periods of normal engine operation.
My copending U.S. patent application Ser. No. 557,397 filed Dec. 2, 1983, which is a continuation-in-part of Application Ser. No. 331,371 filed Dec. 16, 1981 entitled "Improved Engine Prelubricator and Pressurized Lubricant Reservoir", discloses an improved preoiler device having a hollow cylindrical reservoir container, i.e., a reservoir without a piston or a diaphragm. Use of the same solenoid controlled valve assembly disclosed in my U.S. Pat. No. 4,094,293 may be used in conjunction with prelubrication of an internal combustion engine using this hollow reservoir container.
My foregoing copending U.S. patent application Ser. No. 557,397 also discloses that the improved hollow reservoir, i.e., a reservoir without a piston or diaphragm, may be used to supply lubricating fluid to the center bearing of an exhaust-driven turbocharger. No type of valve assembly is used to positively control flow of lubricant from the engine lubricating system to the reservoir or from the reservoir to the center bearing. When the engine is operating normally, the pressurized oil from the engine lubrication system supplies oil both to the center bearing and for charging the reservoir through a T-connection. When the engine shuts down, the reservoir supplies oil for lubricating the center bearing during spin down. A check valve in the T-connection prevents oil from the reservoir from flowing directly to the engine.
The foregoing described center bearing lubrication system works satisfactorily. However, there are disadvantages. Upon the engine start-up, during the time period when the engine is initially building up engine oil pressure, oil flow should go to the engine and the center bearing. However, some of the oil in the foregoing system will go to charging the reservoir which is not desirable during the early start-up period. Once initial start-up has been completed, oil pressure is at a maximum when the engine is warming up and viscosity of the oil is high. However, the foregoing system is unable to retain this maximum pressure in the reservoir because as the engine attains normal operating pressure, the pressure of oil stored in the reservoir will equalize with the normal engine oil pressure.